Flash storage device with read disturb mitigation

ABSTRACT

A method for managing a flash storage device includes initiating a read request and reading requested data from a first storage block of a plurality of storage blocks in the flash storage device based on the read request. The method further includes incrementing a read count for the first storage block and moving the data in the first storage block to an available storage block of the plurality of storage blocks when the read count reaches a first threshold value.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 12/779,001 entitled “FLASH STORAGE DEVICE WITH READDISTURB MITIGATION,” filed on May 12, 2010, which claims the benefit ofU.S. Provisional Application No. 61/177,601, filed on May 12, 2009, thecontents both of which are incorporated by reference in their entiretyherein.

BACKGROUND

The present disclosure concerns flash storage devices and, moreparticularly, flash storage devices configured to mitigate read disturberrors.

Flash memory comprises an array of memory cells. Each memory cellincludes a floating gate transistor that is used to store one or morebits of data. The floating gate transistor in a memory cell isprogrammed by placing an amount of charge on the floating gate, whichincreases the threshold voltage of the transistor. To read the memorycell, a read voltage above the threshold voltage of an unprogrammedtransistor and below the threshold voltage of a programmed transistor isapplied to the control gate of the transistor. An unprogrammedtransistor will conduct at the read voltage, while a programmedtransistor will not. By sensing conduction with the read voltageapplied, the programmed state of the transistor may be read.

NAND flash memory is organized into strings of transistors. Each stringincludes multiple transistors linked together by connecting the sourceof one transistor to the drain of an adjacent transistor. The strings oftransistors typically are organized into physical blocks, each physicalblock comprising a number of pages. The transistors within a stringcorrespond to different respective pages in a physical block. A page isread out of a physical block by applying a read voltage to the controlgate of the transistor corresponding to the particular page in eachstring of the physical block. During a read operation, the othertransistors in the strings are operated in a conducting state by placinga voltage at or above the threshold voltage of a programmed transistoron the respective control gates.

Repeated read operations to a physical block may generate bit errors inone or more pages within the physical block. As noted above, transistorsnot being read are operated in a conducting state during a readoperation. The voltage applied to these transistors may transfer a smallamount of charge to the transistors' floating gates. After a number ofread operations, the cumulative amount of transferred charge may cause apreviously unprogrammed transistor to be incorrectly read as aprogrammed transistor. These errors, referred to as read disturb errors,may result in data loss within physical blocks containing frequentlyread pages unless the flash memory is managed to mitigate read disturberrors and the potential data loss associated with these errors.

SUMMARY

The subject technology mitigates read disturb errors in a flash storagedevice by tracking the number of times data is read from the physicalstorage blocks in the device. The read counts for the physical storageblocks may be compared against a threshold value and preventative actionmay be taken if the read counts reach the threshold value.

According to one aspect of the subject technology, a method for managinga flash storage device is provided. The method includes initiating aread request and reading requested data from a first storage block of aplurality of storage blocks in the flash storage device based on theread request. The method further includes incrementing a read count forthe first storage block and moving the data in the first storage blockto an available storage block of the plurality of storage blocks whenthe read count reaches a first threshold value.

According to another aspect of the subject technology, a flash storagedevice is provided. The flash storage device includes a flash memory anda controller. The controller is configured to initiate a read requestand read requested data from a first storage block of a plurality ofstorage blocks in the flash memory based on the read request. Thecontroller is further configured to increment a read count for the firststorage block and move the data in the first storage block to anavailable storage block in the flash memory when the read count reachesa first threshold value.

It is understood that other configurations of the subject technologywill become readily apparent to those skilled in the art from thefollowing detailed description, wherein various configurations of thesubject technology are shown and described by way of illustration. Aswill be realized, the subject technology is capable of other anddifferent configurations and its several details are capable ofmodification in various other respects, all without departing from thescope of the subject technology. Accordingly, the drawings and detaileddescription are to be regarded as illustrative in nature and not asrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating components of a flash storagedevice according to one aspect of the subject technology.

FIG. 2 is a flowchart illustrating a method for managing a flash storagedevice according to one aspect of the subject technology.

FIG. 3 is a depiction of a virtual-to-physical address mapping tableaccording to one aspect of the subject technology.

DETAILED DESCRIPTION

The detailed description set forth below is intended as a description ofvarious configurations of the subject technology and is not intended torepresent the only configurations in which the subject technology may bepracticed. The appended drawings are incorporated herein and constitutea part of the detailed description. The detailed description includesspecific details for the purpose of providing a thorough understandingof the subject technology. However, it will be apparent to those skilledin the art that the subject technology may be practiced without thesespecific details. In some instances, well-known structures andcomponents are shown in block diagram form in order to avoid obscuringthe concepts of the subject technology.

FIG. 1 is a block diagram illustrating components of a flash storagedevice 10 according to one aspect of the subject technology. As depictedin FIG. 1, flash storage device 10 includes interface 15, controller 20,flash memory 25, and random access memory (RAM) 30. Interface 15facilitates communication of data, commands, and/or control signalsbetween flash storage device 10 and host system 35. Controller 20controls the operation of flash storage device 10 to store and retrievedata in flash memory 25 in accordance with commands received from hostsystem 35. RAM 30 provides temporary storage space for controller 20 toprocess commands and transfer data between host system 35 and flashmemory 25. The operation of each of these components is described inmore detail below.

Interface 15 provides a physical and electrical connection between hostsystem 35 and flash storage device 10. Interface 15 is configured tofacilitate communication of data, commands, and/or control signalsbetween host system 35 and flash storage device 10 via the physical andelectrical connection. The connection and the communications withinterface 15 may be based on a standard interface such as UniversalSerial Bus (USB), Small Computer System Interface (SCSI), SerialAdvanced Technology Attachment (SATA), etc. Alternatively, theconnection and/or communications may be based on a proprietaryinterface. Those skilled in the art will recognize that the subjecttechnology is not limited to any particular type of interface.

Controller 20 manages the flow of data between host system 35 and flashmemory 25. Controller 20 is coupled to interface 15 and is configured toreceive commands and data from host system 35 via interface 15. Forexample, controller 20 may receive data and a write command from hostsystem 35 to write the data in flash memory 25. Controller 20 is furtherconfigured to send data to host system via interface 15. For example,controller 20 may read data from flash memory 25 and send the data tohost system 35 in response to a read command. Controller 20 is furtherconfigured to manage data stored in flash memory 25 and RAM 30 based oninternal control algorithms or other types of commands that may bereceived from host system 35. Those skilled in the art will be familiarwith other conventional operations performed by a controller in a flashstorage device, which will not be described in detail herein.

Controller 20 may be implemented with a general purpose processor,micro-controller, digital signal processor (DSP), an applicationspecific integrated circuit (ASIC), a field programmable gate array(FPGA) or other programmable logic device, discrete gate or transistorlogic device, discrete hardware components, or any combination thereofdesigned and configured to perform the operations and functionsdescribed herein. Controller 20 may perform the operations and functionsdescribed herein by executing one or more sequences of instructionsstored on a machine/computer readable medium. The machine/computerreadable medium may be flash memory 25, RAM 30, or other types ofreadable media from which controller 20 can read instructions or code.For example, flash storage device 10 may include a read only memory(ROM), such as an EPROM or EEPROM, encoded with firmware/softwarecomprising one or more sequences of instructions read and executed bycontroller 20 during the operation of flash storage device 10.

Flash memory 25 represents one or more flash memory devices. The flashmemory devices may all be located on a single flash chip or arranged onmultiple flash chips. The flash memory devices may be split amongmultiple channels through which data is read from and written to theflash memory devices by controller 20, or coupled to a single channel.Flash memory 25 may be implemented using NAND flash, which as discussedabove may result in read disturb data errors with frequent read access.

Flash memory 25 comprises multiple memory cells divided into storageblocks. These storage blocks may be referred to as data blocks or memoryblocks and are addressable by controller 20 using a physical blockaddress. Each of the storage blocks is further divided into multipledata segments or pages addressable by controller 20 using a physicalpage address or offset from a physical block address of the storageblock containing the referenced page. The storage blocks represent theunits of data that are erased within flash memory 25 in a single eraseoperation. The physical pages represent the units of data that are readfrom or written to flash memory 25 in a single read or write operation.Those skilled in the art may recognize other terminology conventionallyused to refer to these data units within a flash memory.

The subject technology is not limited to any particular capacity offlash memory. For example, storage blocks in flash memory 25 may eachcomprise 32, 64, or 128 pages. Additionally, pages in flash memory 25may each comprise 512 bytes, 2 kilobytes, or 4 kilobytes.

RAM 30 represents a volatile memory coupled to and used by controller 20during operation of flash storage device 10. Controller 20 may buffercommands and/or data in RAM 30. Controller 20 also may use RAM 30 tostore address translations tables used to convert logical data addressesused by host system 35 into virtual and/or physical addressescorresponding to portions of flash memory 25. Other types of tables,data, status indicators, etc. conventionally used to manage flashstorage devices also may be stored in RAM 30 by controller 20. RAM 30may be implemented using dynamic random access memory (DRAM), staticrandom access memory (SRAM), or other types of volatile random accessmemory known to those skilled in the art without departing from thescope of the subject technology.

FIG. 2 is a flowchart illustrating a method for managing flash storagedevice 10 according to one aspect of the subject technology. The methodmay be performed by controller 20 reading and executing one or moresequences of instructions from a medium encoded with the instructions.The method begins once controller 20 has completed any start-up tasksand housekeeping procedures required once flash storage device 10 hasbeen powered up or operation interrupted for maintenance purposes and aread request has been received. The read request may be based on acommand received from host system 35 via interface 35. Alternatively,the read request may be generated by an internal process, such as amaintenance process, executed by controller 20. Upon receiving a readrequest, controller 20 initiates the read request in step S201.Initiating the read request may include decoding the request todetermine the address and quantity of data requested. Once the readrequest has been initiated, the process continues to step S202.

In step S202, controller 20 processes the read request. Processing theread request may include translating a logical address in the readrequest, such as a logical address used by host system 35 to identifythe requested data, into a physical address identifying the storageblock and page within flash memory 25 containing the requested data.Address translation also may involve translating the logical address toan intermediate virtual address used by controller 20 to manage the datain flash memory 25, which is subsequently translated to the physicaladdress in flash memory 25. Controller 20 may use one or moretranslation tables stored in RAM 30 to map the logical addressesreferenced by host system 35 into the virtual address and/or physicaladdresses used to identify portions of flash memory 25. Once thephysical address of the requested data is determined by controller 20,appropriate control and addressing signals are applied to flash memory25 to read out the page containing the requested data. Read operationsfor flash memory are well known to those skilled in the art and will notbe described in further detail herein.

According to one aspect of the subject technology, controller 20maintains a read count in RAM 30 for each storage block in flash memory25 containing data. The read count indicates the number of readoperations that have been performed by controller 20 on the respectivestorage block. After controller 20 has read out the page containing therequested data from flash memory 25, controller 20 increments the readcount for the storage block containing the read page in step S203 toreflect another read operation having been performed on the storageblock.

Various data structures may be used by controller 20 to maintain theread counts for the storage blocks in flash memory 25 containing data.For example, FIG. 3 depicts a virtual-to-physical address mapping tableto which an additional column containing read counts has been addedaccording to one aspect of the subject technology. The virtual addressesin the first column of the table (e.g., VBA 0, VBA 1, VBA 2, etc.)represent either the logical addresses referenced by host system 35 oran intermediate virtual address referenced by controller 20 tofacilitate various flash memory management algorithms known to thoseskilled in the art. The physical addresses in the second column of thetable (e.g., PBA 1, PBA 5, PBA 3, etc.) represent the physical addressof the storage block within flash memory 25 containing the datareferenced by the corresponding virtual address in the first column.Address mapping and the various tables used for address mapping are wellknown to those skilled in the art and will not be described in furtherdetail herein.

As noted above, the table depicted in FIG. 3 includes an additionalcolumn containing read counts (e.g., RC 1, RC 5, RC 3, etc.) indicatingthe number of read operations that have been performed on the storageblock identified by the corresponding physical address in the table.Specifically, RC 1 contains a read count for the storage blockidentified by physical address PBA 1, RC 5 contains a read count for thestorage block identified by physical address PBA 5, etc. The read countfor a particular storage block is initialized to zero when the storageblock is first programmed with data after having been erased andincremented each time a page within the storage block is read.

Those skilled in the art will recognize other data structures that maybe used to maintain read counts for the storage blocks within flashmemory 25 without departing from the scope of the subject technology.For example, controller 20 may maintain a read count for every storageblock in flash memory 25 using a dedicated table containing the physicaladdresses of each of the storage blocks and the respective read counts.Alternatively, controller 20 may create a read count for a particularstorage block only after data has been written into one or more pageswithin the storage block.

In step S204 shown in FIG. 2, controller 20 compares the read count ofthe storage block containing the read requested data against a thresholdvalue. If the read count equals the threshold value, the processproceeds to step S205. Otherwise, the process ends until another readrequest is either received in a command from host system 35 or generatedby an internal process within controller 20.

The threshold value represents a number of read counts after which aread disturb error is likely to occur in one or more memory cells withina storage block. The threshold value may be determined based on testingflash memory chips subjected to repeated read operations. Alternatively,the threshold value may be determined by observing the performance offlash storage devices operating with various threshold values or simplysetting the threshold value based on conservative estimates of flashmemory performance. According to one aspect of the subject technologythe threshold value is set at 8192.

According to one aspect of the subject technology, the threshold valueis set at the time of manufacture of flash storage device 10.Alternatively, controller 20 may be configured to set the thresholdvalue based on one or more operational parameters of flash storagedevice 10. For example, controller 20 may reduce the threshold value asflash storage device 10 ages. The older and more used flash storagedevice 10 is, the more likely read disturb errors will occur.

In step S205, controller 20 moves the data contained within the storageblock having the read count equal to the threshold value to an availablestorage block within flash memory 25. The process of moving the datawithin the storage block may include controller 20 marking the storageblock for wear leveling according to one or more wear-levelingalgorithms implemented by controller 20. Wear-leveling algorithms arewell known to those skilled in the art and the subject technology is notlimited to any particular type of wear-leveling algorithm that may beused to identify an available storage block within flash memory 25 andmove data to that storage block. Alternatively, controller 20 mayidentify an available storage block from a group of currently erased(unprogrammed) storage blocks or an available storage block from a groupof storage blocks having sufficient capacity to store the data from thefirst storage block and having a read count below the first thresholdvalue.

As described above, controller 20 maintains the read counts for thestorage blocks in RAM 30, which is volatile memory. Accordingly, ifflash storage device 10 is powered down, the read counts for the storageblocks may be lost. According to one aspect of the subject technology,controller 20 may be configured to take steps to back up the read countsin order to maintain them across power cycles. For example, controller20 may store the read counts in flash memory 25 when flash storagedevice 10 is powered down. Controller 20 may store the read counts alongwith any tables, such as that depicted in FIG. 3, in flash memory 25, ormay store the read counts in reserved portions of the respective storageblocks corresponding to the read counts. When flash storage device 10 ispowered up, controller 20 may then retrieve the read counts for thestorage blocks either be retrieving the stored tables or by populating atable reading the read counts from each storage block.

Controller 20 also may be configured to back up the read counts tonon-volatile memory such as flash memory 25 at other times besidesduring a power down operation. For example, controller 20 may back upthe read counts at regular intervals during operation of flash storagedevice 10. The subject technology is not limited to any particularinterval for backing up the read counts. The intervals may be hourly,daily, weekly, etc. or may be based on monitoring other operationparameters by controller 20. These operation parameters may include thefrequency of commands received from host system 35, the rate ofoperations performed by controller 20, etc.

As noted above, flash memory 25 is organized into physical storageblocks. The storage blocks may be grouped into zones, with each zonecontaining two or more storage blocks. Controller 20 may be configuredto back up the read counts for each of the storage blocks within aparticular zone when the read count of any one of the storage blocksreaches the threshold value causing the data for that storage block tobe moved to an available storage block, or when any of the read countsreach a lower threshold value less than the value triggering a datamove.

Controller 20 may be configured to perform a read check or verifyoperation on each page read from flash memory 25, such as during stepS202 in FIG. 2. For example, controller 20 may verify the data stored ina particular read page using an error-correcting code (ECC) stored inthe page along with the data. If the read check or verify operationfails, controller 20 may move all of the data in the storage blockcontaining the page to an available storage block, such as in step S205in FIG. 2. Controller 20 may move the data in the storage block if theread check indicates a single bit error in the read page. Alternatively,controller 20 may move the data in the storage block if the read checkindicates a number of bit errors greater than a safety countrepresenting a number of errors greater than one up to the maximumnumber of bit errors correctable by the error correction algorithmemployed by controller 20.

Those of skill in the art would appreciate that the various illustrativeblocks, modules, elements, components, methods, and algorithms describedherein may be implemented as electronic hardware, computer software, orcombinations of both. To illustrate this interchangeability of hardwareand software, various illustrative blocks, modules, elements,components, methods, and algorithms have been described above generallyin terms of their functionality. Whether such functionality isimplemented as hardware or software depends upon the particularapplication and design constraints imposed on the overall system.Skilled artisans may implement the described functionality in varyingways for each particular application. Various components and blocks maybe arranged differently (e.g., arranged in a different order, orpartitioned in a different way) all without departing from the scope ofthe subject technology.

It is understood that the specific order or hierarchy of steps in theprocesses disclosed is an illustration of exemplary approaches. Basedupon design preferences, it is understood that the specific order orhierarchy of steps in the processes may be rearranged. Some of the stepsmay be performed simultaneously. The accompanying method claims presentelements of the various steps in a sample order, and are not meant to belimited to the specific order or hierarchy presented.

The previous description is provided to enable any person skilled in theart to practice the various aspects described herein. Variousmodifications to these aspects will be readily apparent to those skilledin the art, and the generic principles defined herein may be applied toother aspects. Thus, the claims are not intended to be limited to theaspects shown herein, but is to be accorded the full scope consistentwith the language claims, wherein reference to an element in thesingular is not intended to mean “one and only one” unless specificallyso stated, but rather “one or more.” Unless specifically statedotherwise, the term “some” refers to one or more. Pronouns in themasculine (e.g., his) include the feminine and neuter gender (e.g., herand its) and vice versa. Headings and subheadings, if any, are used forconvenience only and do not limit the invention.

A phrase such as an “aspect” does not imply that such aspect isessential to the subject technology or that such aspect applies to allconfigurations of the subject technology. A disclosure relating to anaspect may apply to all configurations, or one or more configurations. Aphrase such as an aspect may refer to one or more aspects and viceversa. A phrase such as a “configuration” does not imply that suchconfiguration is essential to the subject technology or that suchconfiguration applies to all configurations of the subject technology. Adisclosure relating to a configuration may apply to all configurations,or one or more configurations. A phrase such a configuration may referto one or more configurations and vice versa.

The word “exemplary” is used herein to mean “serving as an example orillustration.” Any aspect or design described herein as “exemplary” isnot necessarily to be construed as preferred or advantageous over otheraspects or designs.

All structural and functional equivalents to the elements of the variousaspects described throughout this disclosure that are known or latercome to be known to those of ordinary skill in the art are expresslyincorporated herein by reference and are intended to be encompassed bythe claims. Moreover, nothing disclosed herein is intended to bededicated to the public regardless of whether such disclosure isexplicitly recited in the claims. No claim element is to be construedunder the provisions of 35 U.S.C. §112, sixth paragraph, unless theelement is expressly recited using the phrase “means for” or, in thecase of a method claim, the element is recited using the phrase “stepfor.” Furthermore, to the extent that the term “include,” “have,” or thelike is used in the description or the claims, such term is intended tobe inclusive in a manner similar to the term “comprise” as “comprise” isinterpreted when employed as a transitional word in a claim.

What is claimed is:
 1. A method for managing a flash storage device, themethod comprising the steps of: initiating a read request; readingrequested data from a first storage block of a plurality of storageblocks in the flash storage device based on the read request, whereineach of the plurality of storage blocks comprises multiple pages, theplurality of storage blocks in the flash storage device are organizedinto a plurality of zones, each zone comprises at least two storageblocks of the plurality of storage blocks, each storage block has arespective read count, and the first storage block is included in afirst zone of the plurality of zones; incrementing the read count forthe first storage block; determining that the read count for the firststorage block has reached a first threshold value; in response to thedetermination: moving the data in the first storage block to anavailable storage block of the plurality of storage blocks; and backingup the read counts for each storage block in the first zone to anon-volatile memory in the flash storage device; determining that theread count of at least one of the storage blocks in a second zone of theplurality of zones has reached a second threshold value less than thefirst threshold value; and backing up the read counts for each storageblock in the second zone to the non-volatile memory, in response todetermining that the read count of at least one of the storage blocks inthe second zone has reached the second threshold value.
 2. The method ofclaim 1, further comprising maintaining the read count for each of theplurality of storage blocks in the flash storage device containing data.3. The method of claim 2, wherein maintaining the read count for each ofthe plurality of storage blocks containing data comprises storing theread counts in a random access memory in the flash storage device. 4.The method of claim 2, wherein the random access memory comprises adynamic random access memory.
 5. The method of claim 2, whereinmaintaining the read count for each of the plurality of storage blockscontaining data further comprises storing the read counts in thenon-volatile memory in the flash storage device when the flash storagedevice is powered down.
 6. The method of claim 2, wherein maintainingthe read count for each of the plurality of storage blocks containingdata further comprises storing the read counts in the non-volatilememory in the flash storage device at a predetermined interval.
 7. Themethod of claim 1, further comprising: performing a read check on therequested data stored in the first storage block; and moving the data inthe first storage block to an available storage block of the pluralityof storage blocks when the read check fails.
 8. The method of claim 7,wherein performing the read check comprises verifying the data stored inthe first storage block using an error-correcting code in the storeddata, and wherein the read check fails when an error count in theread-checked data is greater than a safety count.
 9. The method of claim1, wherein the available storage block is an erased storage block. 10.The method of claim 1, wherein the available storage block is a secondstorage block of the plurality of storage blocks having a read countless than the first threshold value.
 11. A flash storage devicecomprising: a flash memory; and a controller configured to: initiate aread request; read requested data from a first storage block of aplurality of storage blocks in the flash storage device based on theread request, wherein each of the plurality of storage blocks comprisesmultiple pages, the plurality of storage blocks in the flash storagedevice are organized into a plurality of zones, each zone comprises atleast two storage blocks of the plurality of storage blocks, eachstorage block has a respective read count, and the first storage blockis included in a first zone of the plurality of zones; increment theread count for the first storage block; determine that the read countfor the first storage block has reached a first threshold value; inresponse to the determination that the read count for the first storageblock has reached a first threshold value: move the data in the firststorage block to an available storage block of the plurality of storageblocks; and back up the read counts for each storage block in the firstzone to a non-volatile memory in the flash storage device; determinethat the read count of at least one of the storage blocks in a secondzone of the plurality of zones has reached a second threshold value lessthan the first threshold value; and back up the read counts for eachstorage block in the second zone to the non-volatile memory, in responseto the determination that the read count of at least one of the storageblocks in the second zone has reached the second threshold value. 12.The flash storage device of claim 11, wherein the controller is furtherconfigured to maintain the read count for each of the plurality ofstorage blocks in the flash storage device containing data.
 13. Theflash storage device of claim 12, wherein maintaining the read count foreach of the plurality of storage blocks containing data comprisesstoring the read counts in a random access memory in the flash storagedevice.
 14. The flash storage device of claim 12, wherein the randomaccess memory comprises a dynamic random access memory.
 15. The flashstorage device of claim 12, wherein maintaining the read count for eachof the plurality of storage blocks containing data further comprisesstoring the read counts in the non-volatile memory in the flash storagedevice when the flash storage device is powered down.
 16. The flashstorage device of claim 12, wherein maintaining the read count for eachof the plurality of storage blocks containing data further comprisesstoring the read counts in the non-volatile memory in the flash storagedevice at a predetermined interval.
 17. The flash storage device ofclaim 11, wherein the controller is further configured to: perform aread check on the requested data stored in the first storage block; andmove the data in the first storage block to an available storage blockof the plurality of storage blocks when the read check fails.
 18. Theflash storage device of claim 17, wherein performing the read checkcomprises verifying the data stored in the first storage block using anerror-correcting code in the stored data, and wherein the read checkfails when an error count in the read-checked data is greater than asafety count.